In recent years, governmental authorities and gas turbine manufacturers have become increasingly concerned with pollutant emissions. Gas turbine emission control regulations have been in effect since the early 1970's. Emission regulations have been issued, for example, by the State of California and the United States Environmental Protection Agency. The principal pollutants of concern have been oxides of nitrogen (NO.sub.x) because such oxides are a precursor to air pollution. To comply with applicable regulations, water has been injected into the combustor flame zone to reduce flame temperature Reductions of up to 40 per cent in NO.sub.x resulted, along with low levels of carbon monoxide or unburned hydrocarbon emissions. However, more restrictive emission limitations have led to the requirement of an amount of water nearly equal to the fuel flow to be injected into the combustor flame zone, which in a natural gas fired machine impacts combustion system performance. It is, of course, very important that the combustor be able to burn stably and efficiently. In addition, it is generally a requirement that the combustor design be fired on either gas or oil fuels.
The operation of gas turbine combustors in which the fuel and oxidant are mixed and burned have become more complex in order to achieve the required, and desired, lower emissions. The modern dry low NO.sub.x two-stage premixed combustor is designed for use with natural gas fuel but is capable of operation on liquid fuel with atomizing air The second stage is commonly formed by a venturi, a converging/diverging section, which serves to accelerate the flow exiting the first stage, and which provides a recirculation zone on its downstream face to enhance flame stabilization. The purpose of accelerating the flow from the first stage is to prevent propagation of flame from the second stage into the first stage, so that the combustor can be operated in a premixed mode.
In a premixed mode for natural gas firing, the first stage serves to mix the fuel and air and to deliver a relatively uniform, lean, unburned fuel-air mixture to the second stage. The mixture is sufficiently lean that only a small amount of NO.sub.x is produced, but is also so lean that it will tend to not burn stably and efficiently. Therefore, a portion of the fuel is burned in the second stage nozzle to serve as a piloting source of flame to keep the premixed flow burning. In addition, the recirculation zone on the downstream side of the venturi serves to help stabilize the flame.
In a premixed gas turbine combustor it is desired to provide the following functions, while at the same time also maximizing fuel efficiency and minimizing pollutant emissions:
(1) provide a flow pattern conducive to the mixing of fuel and oxidant prior to the combustion zone, PA1 (2) inhibit the upstream propagation of flame through the fuel-oxidant stream (a phenomenon called flashback), PA1 (3) provide, on the downstream side, flow patterns conducive to flame stabilization, PA1 (4) aid in the atomization and vaporization process of liquid fuel, prior to the combustion process, PA1 (5) provide locations for fuel injection in the midst of an oxidant flow, and PA1 (6) aid in the process of crossfiring between combustors.
The above functions have been accomplished in the past by a plurality, up to three devices. It is desirable to be able to accomplish all of these functions in a single combustor.
In addition, with world fuel supplies and costs being uncertain, the ability to take advantage of a constantly changing fuel market has become an economic necessity. A gas turbine combustor should be designed to operate on a variety of fuels, including gaseous fuels such as natural gas or coal derived gas, and liquid fuels such as light distillate oils.
Various gas turbine combustor and related designs have been developed to achieve low emissions in gas turbine operation. See, for example, U.S. Pat. Nos. 3,958,413; 3,958,416; 3,946,553; 4,292,801 and 4,420,929. Also see the copending patent application, Ser. No. 07/474,394 filed Feb. 5, 1990, by W. T. Bechtel, M. Kuwata and R. M. Washam, and assigned to the same assignee as the present invention. Also see "Gas Turbine Combustion and Emissions" by L. B. Davis, General Electric GE Turbine Reference Library Publication GER-3568, of October, 1988. Also, V-shaped members interposed in the exhaust flow path within aircraft jet engine afterburners or augmenters have been used to provide flame stabilization. However, the processes used heretofore have not optimized all of the above-mentioned considerations and functions.